Method and apparatus for generating satellite tracking information in a compact format

ABSTRACT

Method and apparatus for creating a compact orbit model is described. Satellite tracking data is obtained having a first set of orbit terms that define a first orbit model. The satellite tracking data is formatted to form formatted data having a second set of orbit terms that define a second orbit model. A number of terms in the first set of orbit terms is greater than a number of terms in the second set of orbit terms.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of co-pending U.S. patentapplication Ser. No. 09/915,219, filed Jul. 25, 2001, which incorporatedby reference herein in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to generating satellitetracking information for earth orbiting satellites. More specifically,the invention relates to a method and apparatus for generating satellitetracking information in a first format (e.g., a compact ephemeris model)through a network or communications link, then representing thesatellite tracking information in a second format (e.g., a standardephemeris model) at a receiver.

[0004] 2. Description of the Related Art

[0005] A positioning receiver for the Global Positioning System (GPS)uses measurements from several satellites to compute a position. Theprocess of acquiring the GPS radio signal is enhanced in speed andsensitivity if the GPS receiver has prior access to a model of thesatellite orbit and clock. This model is broadcast by the GPS satellitesand is known as ephemeris or ephemeris information. Each satellitebroadcasts its own ephemeris once every 30 seconds. Once the GPS radiosignal has been acquired, the process of computing position requires theuse of the ephemeris information.

[0006] The broadcast ephemeris information is encoded in a 900 bitmessage within the GPS satellite signal. It is transmitted at a rate of50 bits per second, taking 18 seconds in all for a complete ephemeristransmission. The broadcast ephemeris information is typically valid for2 to 4 hours into the future (from the time of broadcast). Before theend of the period of validity the GPS receiver must obtain a freshbroadcast ephemeris to continue operating correctly and produce anaccurate position. It is always slow (no faster than 18 seconds),frequently difficult, and sometimes impossible (in environments withvery low signal strengths), for a GPS receiver to download an ephemerisfrom a satellite. For these reasons it has long been known that it isadvantageous to send the ephemeris to a GPS receiver by some other meansin lieu of awaiting the transmission from the satellite. U.S. Pat. No.4,445,118, issued Apr. 24, 1984, describes a technique that collectssatellite orbit information at a GPS reference station, and transmitsthe information to the remote GPS receiver via a wireless transmission.This technique of providing the ephemeris, or equivalent data, to a GPSreceiver has become known as “Assisted-GPS”. Since the source ofephemeris in Assisted-GPS is the satellite signal, the ephemerisinformation remains valid for only a few hours. As such, the remote GPSreceiver must periodically connect to a source of ephemeris informationwhether that information is received directly from the satellite or froma wireless transmission. Without such a periodic update, the remote GPSreceiver will not accurately determine position.

[0007] Furthermore, the Assisted-GPS systems typically retransmit theentire ephemeris message to the remote receiver. In many instances,bandwidth or packet size for the transmission of this message is notreadily available.

[0008] Therefore, there is a need for a method and apparatus forproviding satellite trajectory and clock information to a remotereceiver in a compact form.

SUMMARY OF THE INVENTION

[0009] The present invention is a method and apparatus for generatingsatellite tracking data (STD), then transmitting the data to a remotereceiver in a compact form. The STD is derived by receiving at one ormore satellite tracking stations the signals from at least one satelliteand determining satellite tracking information (STI) through signalprocessing or by extracting the ephemeris message from the receivedsignals. STI contains present satellite orbit trajectory data andsatellite clock information.

[0010] The STD is reformatted into a compact format and provided to aremote satellite signal receiver via a network or communications system.The receiver converts the compact format into a standard format and usesthe STD to compute the position of the receiver. The satellite systemmay include the global positioning system (GPS), GLONASS, GALILEO, orother satellite systems that may use STD to enhance the performance ofthe receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] So that the manner in which the above recited features of thepresent invention can be understood in detail, a more particulardescription of the invention, briefly summarized above, may be had byreference to embodiments, some of which are illustrated in the appendeddrawings. It is to be noted, however, that the appended drawingsillustrate only typical embodiments of this invention and are thereforenot to be considered limiting of its scope, for the invention may admitto other equally effective embodiments.

[0012]FIG. 1 depicts a system for creating and distributing satellitetracking data (STD) to remote GPS receivers;

[0013]FIG. 2 depicts a flow diagram of a method for forming the STD fromthe satellite measurements made at satellite tracking stations;

[0014]FIG. 3 depicts a flow diagram of a method for forming a compactorbit model in accordance with the present invention; and

[0015]FIG. 4 depicts an example of compacting the orbit model, where twoorbit model terms are compacted into a single term.

[0016] To facilitate understanding, identical reference numerals havebeen used, wherever possible, to designate identical elements that arecommon to the figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017]FIG. 1 depicts a block diagram of a system 100 for creating anddistributing satellite tracking data (STD). The satellite system mayinclude the global positioning system (GPS), GLONASS, GALILEO, or othersatellite systems that may use STD to enhance the performance of thereceiver. The following disclosure uses GPS as an illustrative systemwithin which the invention operates. From the following disclosure,those skilled in the art will be able to practice the invention inconjunction with other satellite based positioning systems.

[0018] A network of GPS tracking stations 102 is used to collectmeasurement data from the GPS satellites 104. Such a network isdescribed in detail in U.S. Pat. No. 6,411,892, issued Jun. 25, 2002,which is incorporated herein by reference. The network could compriseseveral tracking stations that collect satellite tracking information(STI) from all the satellites in the constellation, or a few trackingstations, or a single tracking station that only collects STI for aparticular region of the world. An STD collection and computation server106 collects and processes the measurement data (this measurement datais referred to herein as satellite tracking information (STI) thatincludes at least one of: code phase measurements, carrier phasemeasurements, Doppler measurements, or ephemeris data). The ephemerisdata may be the decoded ephemeris message extracted from the GPS signalitself.

[0019] The server may create long term STD in accordance with theteachings of U.S. Pat. No. 6,542,820, issued Apr. 1, 2003, or standardephemeris message data in accordance with the teachings of U.S. Pat. No.5,365,450, issued Nov. 15, 1994, both of which are incorporated hereinby reference. The server 106 may produce one or more of thefollowing: 1) accurate satellite tracking data (STD) (e.g., a trajectoryof each satellite and/or a clock offset measurement) during the datacollection period, 2) a prediction of the future STD of each satellite,and 3) models that match the future STD of each satellite.

[0020] The server 106 comprises a central processing unit (CPU) 118,support circuits 122, and memory 120. The CPU 118 may be any one of themany CPUs available on the market to perform general computing.Alternatively, the CPU may be a specific purpose processor such as anapplication specific integrated circuit (ASIC) that is designed toprocess satellite tracking information. The support circuits 122 arewell known circuits such as clock circuits, cache, power supplies andthe like. The memory 120 may be read only memory, random access memory,disk drive storage, removable storage or any combination thereof. Thememory 120 stores executable software, e.g., STD software 124, that,when executed by the CPU 118, causes the system 100 to operate inaccordance with the present invention.

[0021] The set of satellite trajectory and clock data produced by theSTD software 124. The STD is stored in an STD database 108. Adistribution server 110 accesses the database 108 to gather the mostrecent set of STD, formats the data using the formatting software 111,and distributes the formatted data to GPS devices 112 that requiresatellite orbit information. The software 111 produces a compact format,e.g., a compact ephemeris model, in accordance with the presentinvention.

[0022] The distribution process may be implemented using some form ofwireless communications system 114, or over the Internet 116, or acombination of both, or by some other means of communication. Once theGPS devices 112 have received the compact ephemeris model, they expandthe model to a format that is conventional for receiver. The compactephemeris model distributed to the GPS devices may be in a similarformat as the broadcast ephemeris or may be some other model format thatis defined by the GPS device. Herein this orbit data is generallyreferred to as a satellite tracking model (STM). The loading of the STMinto the GPS receiver can be accomplished in many ways. Using the cradlefor a personal digital assistant (PDA), direct connection to a network,or a wireless technology, such as Bluetooth or a cellular network, are afew examples of how the satellite data can be transferred to thereceiver. The transmission is generally accomplished by broadcasting acompact model of the STD (or a compact model representing a portion ofthe STD) without knowledge of the specific location of the GPS receiver.As such, the distribution server does not require the GPS receiver tosend any information through the network to the distribution server.

[0023] Since GPS is a ranging system in and of itself, the datatransmitted by the GPS satellites can be used to determine the range,range-rate and clock offsets to the GPS satellites from a set oftracking stations. This set of observations generated by the trackingstations 102 is used in the orbit determination process, and in theestimation of the satellite clock characteristics. The set of monitoringstations 102 could be a single station, a public network such as theContinuously Operating Reference System (CORS), or a privately ownedand/or operated network.

[0024]FIG. 2 depicts a flow diagram of the process 200 of the presentinvention. The process 200 begins at step 202, wherein the satellitemeasurements are collected at the tracking stations. At step 204, thesatellite trajectory data (STD) is computed or extracted from thesatellite signals. The STD is then stored at step 206 in the STDdatabase. At step 208, the database is accessed and the formattingsoftware is executed to convert the formatting of the accessed STD. Theformatted STD is output as the compact model at step 210.

[0025] One embodiment of the invention formats the STD as a subset ofthe standard ephemeris parameters defined in ICD-GPS-200c. Fitting theSTD to the desired compact orbit model can be accomplished in a numberof mathematical methods. The preferred embodiment is a least-squares fitof the orbit model parameters to the trajectory data. Other methods,such as Kalman filters or other estimators can also be used to obtainthe orbit model parameters that best fit the trajectory data. Thesetechniques of fitting data to orbit models are well known to peopleskilled in the art of orbit determination and orbit modeling.

[0026] The least squares technique provides an optimal fit of thetrajectory data to the model trajectory formed from the compact orbitmodel parameters. FIG. 3 depicts a flow diagram of a method ofgenerating an orbit model using a least squares estimation technique.

[0027] At step 302, the STD for the desired time interval is extractedfrom the STD database. The orbit model parameters are initialized to theorbit model values obtained by a similar process for the previousinterval. This guarantees that the initial orbit model parameters are agood fit at least for the beginning of the desired time interval. Therest of the process 300 will ensure that the parameters are adjusted sothat they become a good fit for the entire time interval.

[0028] In the preferred embodiment there are 15 orbital parameters to beadjusted:

[0029] Square root of semi-major axis (meters{circumflex over ( )}½)

[0030] Eccentricity (dimensionless)

[0031] Amplitude of sine harmonic correction term to the orbit radius(meters)

[0032] Amplitude of cosine harmonic correction term to the orbit radius(meters)

[0033] Mean motion difference from computed value (radians/sec)

[0034] Mean anomaly at reference time (radians) Amplitude of cosineharmonic correction term to the argument of latitude (radians)

[0035] Amplitude of sine harmonic correction term to the argument oflatitude (radians)

[0036] Amplitude of cosine harmonic correction term to the angle ofinclination (radians)

[0037] Amplitude of sine harmonic correction term to the angle ofinclination (radians)

[0038] Longitude of ascending node of orbit plane at weekly epoch(radians)

[0039] Inclination angle at reference time (radians)

[0040] Rate of inclination angle (radians/sec)

[0041] Argument of perigee (radians)

[0042] Rate of right ascension (radians/sec)

[0043] At step 303, some of the terms in the 15 term set are set tozero. The terms that are selected are the 6 harmonic terms such thatthere are 9 remaining parameters. This approach is particularly usefulwhen bandwidth and/or packet size is limited in the communication linkthat will be used to convey the orbit model to the satellite signalreceiver, e.g., the remote GPS receiver. The subset of 9 parameters, bysetting all harmonic terms in the model to zero, is:

[0044] Square root of semi-major axis (meters{circumflex over ( )}½)

[0045] Eccentricity (dimensionless)

[0046] Mean motion difference from computed value (radians/sec)

[0047] Mean anomaly at reference time (radians)

[0048] Longitude of ascending node of orbit plane at weekly epoch(radians)

[0049] Inclination angle at reference time (radians)

[0050] Rate of inclination angle (radians/sec)

[0051] Argument of perigee (radians)

[0052] Rate of right ascension (radians/sec)

[0053] The receiver can then reconstruct a standard ephemeris model bysetting the “missing” harmonic terms to zero. In essence, the receiverreformats the STD for processing by the receiver circuits.

[0054] As an example of the method of generating the compact model,consider FIG. 4, which shows, for simplicity, just two terms of an orbit400: an orbital radius (A), and a radial harmonic term (r). For thissimple example, these two terms form the non-compact model, wherein theorbit is described by a circle of radius (A) plus a harmonicperturbation (r). To produce a more compact model that fits the actualorbit over an interval 402, the method of the invention removes theharmonic term (i.e., sets the term (r) to zero) and increases theorbital radius (A) to a larger value (A1). The compact model is an orbitdescribed by a circle with radius A1. If an application requires anon-compact orbit model, then the compact model (A1) can be representedas a non-compact model by specifying a harmonic term (r1) equal to zero.This compact model will fit the original orbit, over an interval 402,with a small error.

[0055] In the preferred embodiment, 6 harmonic terms are removed fromthe 15-parameter model, and the other 9 terms are adjusted by process300 that is analogous to the example 400 to provide a compact model thatis accurate over a pre-defined interval. By adjusting the 9 remainingterms of an orbit model, while “zeroing” 6 harmonic terms, the compactmodel can be made accurate over a period of time such that a GPSreceiver that relies on a compact model to compute position wouldcompute a location that is no more than 2 meters less accurate than ifthe receiver used a full orbit model to compute position.

[0056] There are many alternative embodiments that will be readilyapparent to those skilled in the art, such as removing more or fewerterms before adjusting the remaining terms, setting removed terms tosome value other than zero, and defining new terms that model the orbit.

[0057] Returning to FIG. 3, at step 304, the orbit model is used topredict what the trajectory would be, the predicted data is denoted the“Model Trajectory Data” (MTD). If the model were perfect, the MTD wouldcoincide exactly with the STD. At step 306, the MTD and STD are comparedto see how closely the orbit model fits the orbit data. In the preferredembodiment, the comparison step 306 is performed by summing the squaresof the differences between each trajectory point in the STD and thecorresponding point in the MTD, and comparing the resulting sum to athreshold. If the fit is “good”, the model parameters are deemed “good”and the process stops at step 310. If the fit is not good then the modelparameters are adjusted at step 308. There are many techniques wellknown in the art for adjusting model parameters to fit data. Steps 304,306 and 308 are repeated until the model parameters are found that fitthe STD well.

[0058] There are a large number of alternative embodiments to reduce thesize of the data, i.e., compacting the STD, while still providing amodel that fits the STD, including:

[0059] Removing parameters from the model, and replacing them with aconstant, such as zero—as done above—or some other predetermined value,which is either stored in the Remote GPS Receiver, or occasionally sentto the receiver. The predetermined value may be determined by a GPSalmanac stored at both the receiver, and the distribution server.

[0060] The resolution of the parameters may be restricted in the process300, this too reduces the amount of data that must be sent to the mobileGPS receiver.

[0061] Parameters, which are similar among two or more satellites, maybe represented as a master value plus a delta, where the delta requiresfewer bits to encode; an example of this is the parameter Eccentricity,which changes very little among different GPS satellites.

[0062] Some of these approaches reduce the ability of the model to fitthe data over a period of time (e.g., four hours). In this case, the fitinterval may be reduced (e.g. to two hours) to compensate. The accuracyof fit of the model can be traded off against the period of time overwhich the model is valid.

[0063] While the foregoing is directed to embodiments of the presentinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of creating a compact orbit model, comprising: obtainingsatellite tracking data having a first set of orbit terms that define afirst orbit model; and formatting said satellite tracking data to formformatted data having a second set of orbit terms that define a secondorbit model, where a number of terms in said first set of orbit terms isgreater than a number of terms in said second set of orbit terms.
 2. Themethod of claim 1, wherein the obtaining step comprises: receivingsatellite signals from a least one receiving station; and extractingmeasurement data from said satellite signals; and forming the satellitetracking data in response to the measurement data.
 3. The method ofclaim 2, wherein the measurement data comprises at least one of: codephase measurements, carrier phase measurements, Doppler measurements,and satellite ephemeris data.
 4. The method of claim 1, wherein saidsatellite tracking data comprises at least one of a satellite orbitmodel or a satellite clock model.
 5. The method of claim 1, wherein saidsatellite tracking data comprises at least one of: data representativeof a satellite orbit model, an orbit model, and data representative of asatellite clock model.
 6. The method of claim 1, where said terms insaid second orbit model require fewer bits to encode it than said termsin said first orbit model.
 7. The method of claim 1, wherein saidsatellite signals are GPS signals.
 8. The method of claim 1, where anaccuracy of the data in said second orbit model is increased bydecreasing a time interval represented by said formatted data definingsaid second orbit model.
 9. The method of claim 1, wherein saidformatting step further comprises: zeroing a plurality of terms in saidfirst set of orbit terms.
 10. The method of claim 9, wherein saidformatting step further comprises: adjusting a plurality of non-zeroterms in said second set of orbit terms in response to the effects ofzeroing terms in said first set of terms.
 11. The method of claim 9,wherein the plurality of terms in said first set of orbit terms that arezeroed are harmonic terms.
 12. The method of claim 1, wherein saidformatting step further comprises: replacing a plurality of terms insaid first set of orbit terms with a constant value.
 13. The method ofclaim 12, wherein the constant value is determined in response tosatellite almanac data.
 14. The method of claim 1, wherein saidformatted data having said first set of orbit terms comprises parametersdefined in ICD-GPS-200.
 15. The method of claim 1, wherein at least oneterm of said second set of orbit terms is defined as a number with alower resolution than a corresponding term in said first set of orbitterms.
 16. A method of creating a compact orbit model, comprising:receiving satellite signals having satellite tracking data from at leastone receiving station; extracting at least a portion of the satellitetracking data from the satellite signals, where said portion comprises afirst set of orbit terms that define a first orbit model; and formattingsaid portion to form formatted data having a second set of orbit termsthat define a second orbit model, where a number of terms in said firstset of orbit terms is greater than a number of terms in said second setof orbit terms.
 17. The method of claim 16, wherein said satellitetracking data comprises at least one of a satellite orbit model or asatellite clock model.
 18. The method of claim 16, wherein saidsatellite tracking data comprises at least one of: data representativeof a satellite orbit model, an orbit model, and data representative of asatellite clock model.
 19. The method of claim 16, where said terms insaid second orbit model require fewer bits to encode it than said termsin said first orbit model.
 20. The method of claim 16, wherein saidsatellite signals are GPS signals.
 21. The method of claim 16, whereinsaid satellite tracking data comprises at least one of code phasemeasurements, carrier phase measurements, Doppler measurements, andsatellite ephemeris data.
 22. The method of claim 16, where an accuracyof the data in said second orbit model is increased by decreasing a timeinterval represented by said formatted data defining said second orbitmodel.
 23. The method of claim 16, wherein said formatting step furthercomprises: zeroing a plurality of terms in said first set of orbitterms.
 24. The method of claim 23, wherein said formatting step furthercomprises: adjusting a plurality of non-zero terms in said second set oforbit terms in response to the effects of zeroing terms in said firstset of terms.
 25. The method of claim 23, wherein the plurality of termsin said first set of orbit terms that are zeroed are harmonic terms. 26.The method of claim 16, wherein said formatting step further comprises:replacing a plurality of terms in said first set of orbit terms with aconstant value.
 27. The method of claim 26, wherein the constant valueis determined in response to satellite almanac data.
 28. The method ofclaim 16, wherein said formatted data having said first set of orbitterms comprises parameters defined in ICD-GPS-200.
 29. The method ofclaim 16, wherein at least one term of said second set of orbit terms isdefined as a number with a lower resolution than a corresponding term insaid first set of orbit terms.
 30. An apparatus for distributing compactsatellite orbit models, comprising: a database for storing satellitetracking data having a first set of terms that define a first orbitmodel; and means for formatting said satellite tracking data to formformatted data having a second set of orbit terms that define a secondorbit model, where a number of terms in said first set of orbit terms isgreater than a number of terms in said second set of orbit terms. 31.The apparatus of claim 30, further comprising: at least one satellitesignal receiver for receiving satellite signals from at least onesatellite; means for extracting measurement data from said satellitesignals; and means for forming the satellite tracking data in responseto the measurement data.